older times on Venus have been lost.
Less than 1 billion years ago, the geology of Venus seems to have
undergone a global pulse of rapid activity and then slowed to a crawl.
This is disturbing. Venus on the inside should be just like Earth. Earth
is stable and predictable. Isn’t it? A billion years isn’t all that long ago
in planetary time. Why didn’t Venus settle down a long time ago, as
Earth did? Is there something we should know?
The answer may be that Venus oscillates every half billion years or so
between spurts of furious global volcanic activity and long spells of rel-
ative inactivity. Instead of the smoothly running heat engine of plate
tectonics that our planet enjoys, the internal engine of Venus might run
in brief dramatic fits of intense activity. Like a motor, seriously in need
of lubrication, that keeps getting stuck, the global style of Venus might
be to occasionally lurch into action in massive geological tantrums that
renew the entire surface all at once, letting a blast of heat out of the
interior. If this fits-and-starts alternative to Earth’s plate tectonics really
does occur on Venus, we have to ask, “Why the difference?” Since ther-
mal evolution, I’ve led you to believe, is controlled by planetary size,
shouldn’t Venus and Earth have the same overall behavior?
It may all come back, once again, to location, and the drying of
Venus by the nearby Sun. The above analogy, of Earth being a well-
oiled machine and Venus one in need of a lube job, may not be too far
from the truth. In this case, though, it is water, not oil, providing
the lubrication. Earth is so soggy that a large portion of the rocks in the
crust and mantle are hydrated. A layer of water-softened rocks at the
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Image unavailable for
electronic edition
base of Earth’s crust allows the tectonic plates to slide around the sur-
face of our planet nice and easy, smooth and slow. Models of Earth’s
plate tectonics, when altered to simulate dry rocks instead of the real
waterlogged ones, begin to seize up and, instead of running smoothly,
stop and start like a backfiring jalopy. Dehydrate Earth and it may
begin to act like Venus.
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Maybe then, the capacity for Earth-style plate tectonics on Venus
was lost along with the water in a runaway greenhouse. If this picture is
accurate, the lightly sleeping monster of Venusian geology may be get-
ting ready to stir again. Pay attention, because the fireworks could start
anytime in the next couple of hundred million years.
Actually, there is good evidence that the monster is not fully asleep.
There are abundant atmospheric signs of ongoing volcanic activity. The
mixture of gases in the Venusian air is “out of equilibrium” with the
minerals at the surface. This means chemicals are primed to react, itch-
ing to get at each other. When we see such a condition in a planet’s
atmosphere, it’s a clue that something is up. An atmosphere out of equi-
librium is like a pile of hungry cats in a roomful of freshly opened cans
of sardines. The situation is not static. A disequilibrium condition does
not last long unless some energetic process is actively keeping it that
way (a constant supply of fresh sardines, for instance, could explain
why the cats haven’t eaten them all). A disequilibrium mixture of gases,
left to itself, would rapidly undergo chemical reactions and change into
a different mix, more in equilibrium. So the atmosphere of Venus has
not been left to itself. Something is regularly injecting fresh, reactive
gases into the air. In particular, the amount of SO2 is suspiciously high.
This is strong circumstantial evidence of currently active volcanoes.
The most obvious, visible sign of something actively disturbing the
atmosphere of Venus is the global clouds themselves. In an extreme
form of acid rain, gases spewing from Venus’s volcanoes are actively
maintaining the sulfuric clouds and supporting the intense greenhouse
climate. Without a continuous source of fresh sulfur gases from active
volcanoes, the clouds of Venus would disappear in a mere 30 million
years, as sulfur was consumed by reactions with surface rocks. The
bright clouds of Venus are the smoking gun of active volcanoes on the
surface in the geologically recent past.
From this, and the fresh appearance of the largest volcanoes in the
Magellan radar pictures, many of us believe that Venus today has active
volcanism. Yet most of the surface seems to have been formed during a
bygone era when volcanic activity was much more intense. Since we’ve
found that the current climate and clouds are strongly affected by the
paltry amount of volcanism occurring now, this leads us to ask what
the climate of Venus was like 600 million years ago when the global
volcanic plains were forming, when lava was gushing onto the surface,
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and greenhouse gases were pouring out of the ground at hundreds of
times their modern rates.
Inspired by our mentor Jim Pollack, my collaborator Mark Bullock
and I have taken on this problem. It’s not easy to tackle because it
involves combining numerous techniques that have previously been
handled by several subdisciplines that haven’t always played well
together. To follow the changing environment of Venus we need to be
climate modelers, cloud physicists, atmospheric chemists, and volcanol-
ogists. No one can do all of this, but we’ve had lots of help.*
First we needed a good climate model, better than any previously
constructed for Venus. We had to accurately simulate the numerous
transitions of energy that occur when sunlight reaches Venus, reflecting
off the clouds, filtering through the atmosphere, warming the surface,
and reradiating as infrared, which heats the air. But, simulating the heat
balance in the present atmosphere was only the starting point. We also
need to be able to change the mix of gases in our model, simulating an
episode of enhanced volcanic gases, and calculate the changes in sur-
face temperature, cloud structure, chemistry, and so on. Fortunately, we
had Jim Pollack to help us design the initial version of our climate
model.
Pollack was Carl Sagan’s first grad student at Harvard in the 1960s,
and he cut his teeth on early climate models of Venus. For several
decades after that, he oversaw an army of researchers at NASA’s Ames
Research Center in Silicon Valley and cranked out dozens of important
papers on an astonishing range of planetary topics. One of his passions
was climate evolution on Earth-like planets.
My first job after grad school was as a postdoctoral researcher at
NASA Ames, with Jim Pollack as my adviser. Jim had a way of cutting
through to the core of a scientific problem and helping you see clearly
what needed to be done to solve it.
The major project Jim and I worked on during my apprenticeship
with him was the construction of an improved model of
the Venusian
clouds. For this we used some wonderful infrared snapshots that the
Galileo spacecraft had taken as it flew close by the night side of Venus
*As John Lewis says, twisting the “standing on the shoulders of Giants” line of Newton, we’ve been stepping on the ankles of midgets.
Venus and Mars
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in February 1990, on the first leg of its wayward six-year journey to
Jupiter. We used the pattern of the heat leaking unevenly through the
clouds to nail down their structure and composition.
During this same period Magellan was in orbit around Venus and
the first global surface maps were being assembled. The strange crater
distribution suggested immediately that Venus must have had periods
of intense volcanic activity. Jim was a global thinker and he encour-
aged the same in his colleagues—we had wonderful conversations
about what this strange surface history might have meant for the
atmosphere, clouds, and climate. He was a rigorous scientist who
wasn’t afraid to think bold thoughts—he encouraged me to follow my
ideas, however fantastic, as long as I could back them up with physical
models.
Around the time I was finishing my postdoc gig and packing up the
old Corolla for the move to Colorado, Jim became ill with a rare form
of cancer. When he became too sick to go to the office, he hooked up
his first home computer and continued to hold court in cyberspace.
Like a shark that needs to keep swimming, Jim needed to do science.
He kept up his input on numerous ongoing projects, barely ever hinting
at his worsening condition unless directly asked about it, right until his
untimely death in June 1994 at the age of fifty-five. I’ve kept several of
my final e-mails from him: strange electronic relics of a great mind and
a kind soul.
Starting with the climate model Jim helped us to map out, Mark and
I have added parts that simulate volcanic emissions of gas to the atmo-
sphere, chemical reactions between gases in the atmosphere and miner-
als on the surface, diffusion of gases into the crust, the formation and
destruction of clouds, and the escape of gases into space.
What we’ve found is that as the geology of Venus has gone through
intense oscillations in activity, the climate has followed suit, episodi-
cally undergoing hundreds of degrees of global cooling, and then
warming. These extreme temperature changes should have made sur-
face rocks expand and contract, causing Venusquakes around the
planet. In fact, in Magellan images we think we see the signs of climatically induced surface wrinkles that formed suddenly all around the
planet after the epoch of massive volcanic outpourings. Unexpectedly,
climate modeling may have helped solve some mysteries of Venusian
surface geology.
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S U R V I V O R
When we look into the details of climate and geological evolution on
Venus, we discover an interconnected maze of ongoing processes, all
changing and mutually influencing one another. Volcanoes on the surface
alter the atmosphere and clouds. This changes the climate, which affects
the surface geology and chemistry and even influences the planet’s interior.
In turn, these internal changes eventually influence the rate of volcanism.
In this sense, we’ve been learning that Venus, despite having a surface so
hot that it glows at night and an atmosphere that has quickly consumed
every spacecraft we’ve dropped in there, is actually quite Earth-like in some
profound ways. As extreme and hostile as the environment there seems to
us, it represents a delicate and subtle balance of ongoing geological, mete-
orological, and climatic activity. Much planetary exploration involves
studying dead worlds, surveying places that were once active but have long
been still, and trying to reconstruct the events of billions of years ago.
Venus and Earth are in a class of their own. They are both survivors.
In terms of its geology and climate, Venus, like Earth, is still alive and
kicking. The rampant disequilibrium and complex climate cycles of the
type we have found on Venus are often thought to be the hallmarks of
planets with life. But the idea of life on Venus is not taken seriously
because of our understandable obsession with water. Later, in chapter
17, I’ll take a critical look at this consensus conclusion, as I consider a
new way of thinking about life on planets. I’ll ask whether life, rather
than being something that happens on planets, might be more properly
viewed as something that happens to planets.
Our interconnected “systems” approach to studying Venus has
helped us develop an arsenal of modeling techniques we are now direct-
ing toward the general processes of terrestrial planet evolution any-
where in the galaxy or beyond. We wish to explore the balance of
chance and determinism that goes into shaping worlds, in order to
know how much we can safely generalize from the local examples.
Understanding planetary evolution and the different paths it can take is
a vital step in comprehending this universe’s potential to make life.
I N T H E Z O N E
Earth’s biosphere can be seen as an extension of our oceans, as a capac-
ity that they have achieved, with ourselves as the semisentient ocean’s
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primitive thought organs. So, though we run the risk of geocentric nar-
rowness anytime we define specific criteria for finding life elsewhere,
naturally we focus on searching for liquid water.
One key factor in maintaining a liquid water biosphere on a planet,
over billions of years, is its distance from the Sun, or whatever star it
may be orbiting. If it is in too close, then, like Venus, its oceans will boil
away. At too great a distance a water world will at best have icy polar
caps on the surface.
When I lived in Tucson, I loved the winding drive up to the observa-
tory on Mount Lemmon, north of town, where grad students escape
the summer heat and learn to use large telescopes. As you ascend from
the hot town to the cool mountaintop, you pass through several distinct
ecological zones, each existing only within a certain range of tempera-
ture and moisture and containing distinct species of life. You start off in
low desert full of giant saguaro cacti, then ascend through mesquite
grasslands, pine-oak woodlands, and ponderosa pines, and end up in a
dense, lush forest of tall fir and spruce.
Does the solar system have a well-defined ecological zone, a range of
distance from the Sun, where liquid water is stable? If so, the inner and
outer edges of this zone are slowly drifting away from the Sun as our
star ages and heats up. Earth is probably safe for another 1 or 2 billion
years. After that, we will go the way of Venus, our oceans boiling off
and fleeing back to space as the inner, hot edge of the “habitable zone”
sweeps outward and leaves our spent planet behind.
My colleagues and I are now pursuing research to define the habit-
able zones around other stars. A zon
e of water-based life may be a com-
mon feature surrounding stars. Even if other, non-water-based chemical
structures, unknown to us, can support life, it is reasonable to expect,
given the strong dependence of chemistry on temperature, that each
kind of life will have a limited temperature range within which it can
thrive. Such alternate biospheres might have their own habitable zones
at different distances from the same star. Traveling out through a solar
system, from hot star to distant, cold edge, you may pass through sev-
eral ecological zones, just as you do on the drive up Mount Lemmon to
observe the planets and stars.
Assuming for now that water is what life needs, then the divergent
lives of Venus and Earth can teach us where to expect the inner hot
edge of the habitable zone for water-based life anywhere in the uni-
verse. What about the outer, cold edge? Is Mars inside or outside the
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habitable zone? I.e., is there liquid water on the surface? That seems to
depend on when you look.
R U S T I N P E A C E
On the afternoon of August 8, 1996, I was a blissed-out, naked, buoy-
ant extremophile, floating in the 103-degree waters of Orvis Hot
Springs in Ridgway, Colorado. Among the relaxed, random chatter of
other naked floaters I thought I heard a woman casually mention that
she had just heard on the radio that “they” had discovered life on
Mars. Her comment did not seem all that remarkable, you must under-
stand, because this is the kind of thing you hear all the time if you hang
around hot springs in southwestern Colorado, and after a while you
learn to calibrate your responses.
Later that day, however, when we went into town in search of food,
there it was, screaming from every newspaper box: the banner headline
“Life Found on Mars!” “Holy shit,” I mumbled, fumbling in my
pocket for fifty cents. That’ll teach me to hide from e-mail for a few
days.
The discovery concerned a meteorite that had been found in
Antarctica in 1984 and determined to be from Mars. This part wasn’t
new—we’ve known for over a decade that more than a dozen rocks in
our meteorite collections come from the Red Planet.
Now, however, at a press conference held at NASA headquarters in
Washington, a group of reputable scientists had announced with great
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